Add integration test for `backup` Docker image

[MoritzWeber0]

In Capella 6.0.0, the arguments of the importer script have been changed. Thus, the backups fail as we haven't noticed it. We need a full integration test.

Add a new pytest for the backup job:

• [x] Spawn the T4C server in the pytest
• [ ] Populate the server with an existing project. The data has to be placed in a specific location, e.g. t4c/server/tests/data/$CAPELLA_VERSION
• [x] Start a Git server. The git http-backend command might be helpful.
• [ ] Run the backup Docker container
• [ ] Check if the Git server contains the project with all important files.

In addition, the backup image should have support for Capella 6.0.0:

• [ ] Fix the backup Dockerfile / Python script to support Capella 6.0.0.

[MoritzWeber0]

Implemented in https://github.com/DSD-DBS/capella-dockerimages/pull/82